With an increase in the complexity of arrangement of parts mounted on the surface of electric wiring boards, for example, of printed circuit boards, electric parts such as integrated semiconductor devices are mounted in a bare state directly on printed boards and electrically connected thereto. In this case, the electric parts mounted in a bare state on the printed board must be sealed with an electrically insulating resin mainly so as to retain their quality.
As such a method of sealing electric parts with resin, the so-called potting method has already been proposed wherein a pneumatically operable dispenser a is used for applying a liquid sealing resin d dropwise onto an electric part c mounted on a printed board b to seal the part as shown in FIG. 10. The potting method has the problems to be described below with reference to FIGS. 11 and 12.
(1) As seen in FIG. 12, the resin portion d' sealing the electric part c appears convex in shape, has a thickness which is largest at the center and gradually decreases toward the outer periphery, and therefore fails to produce a uniform sealing effect.
(2) The part c is usually square to rectangular, whereas the sealing resin portion d' is circular as shown in FIG. 11, so that the part c must be sealed over an excessive area. This is not desirable in view of the high complexity of the arrangement on the board.
(3) Although the amount of resin d to be applied dropwise can be suitably determined according to the size of the part c, the resin d applied is difficult to control in thickness.
(4) For application to LSIs or like large-scale electric parts, the resin d must be applied dividedly several times. It is then difficult to give a uniform appearance and a uniform thickness to the sealing resin layer to be formed. The part therefore can not be sealed effectively with good stability.
(5) The method is low in productivity since parts c must be sealed individually one by one.
(6) When the resin is to be applied to the next part after application to one part, cobwebbing of the resin is liable to occur. The thread of resin is then likely to soil the printed board b.
We have already proposed a method of sealing electric parts mounted on the surface of electric wiring boards with resin using screen printing means (see, for example, Unexamined Japanese Patent Publication No.82639/1989). All the foregoing problems (1) to (6) involved in the potting method can be overcome by this method which employs the screen printing means.
The proposed sealing method with use of the screen printing means employs a screen having apertures in the same pattern as the electric parts mounted on the board and is practiced by forcing a liquid sealing resin serving as the printing ink into a space formed between each of the apertures and the part registered therewith by the operation of a squeegee to fill the space and seal the part. However, when the printing cycle is repeated many times, the liquid sealing resin serving as the printing ink adheres to the rear side of the screen, and the resin portion adhering to the rear side is printed on the surface of the board in the form of a thin overflow portion around the sealing resin layer, consequently collapsing the resin layer slightly as will be described below in detail.
Further when the liquid sealing resin is forcibly filled into the screen aperture by operating the squeegee, the amount of resin filled in is larger toward the end side of stroke of the squeegee than toward the starting side of the stroke. The uneven amount of resin filled in gives an uneven thickness to the sealing resin layer and results in reduced productivity as will be described in detail later.